Positioning of the lens of the look-down zone is critical to look-down performance. Typically, a PIR will have its look-down lens as an integral part of the outside housing, as seen on the left side of the figure. This creates two problems:
1. Detection can be compromised. A lens focuses its infrared energy at a fixed spot just like a magnifying glass or telescope. For best operation, this fixed spot in a motion detector is on the pyroelectric elements. When the PCB is moved vertically for coverage adjustment, this energy will be defocused except when the board is in just one particular position. Less energy is collected and detection performance suffers.
2. Immunity to rodents is compromised. The long focal length (the distance from the lens to the pyro elements) causes the projected beam for the look-down zone to be small. As seen in the left figure, rodents can fill a large portion of the beam and generate a false alarm.
ADEMCO look-down designs overcome both problems by using a shorter focal length lens mounted very close to the detector as part of the circuit board, as shown on the right figure. When the PCB is shifted vertically for pattern coverage adjustment, infrared energy is always focused since the distance between the lens and the elements never changes. Because the lens is close to the element, the projected beam on the floor is now large compared to the size of a rodent. Avoid both detection and false alarm problems with ADEMCO look-down designs.